a. The longest chain should be identified, in this case longest chain has 7 carbons hence the name of parent chain heptane.

b. Then the substituents have to be identified, here we have 3 substituents (One ethyl group and two methyl groups). When we start numbering the parent chain it is from the side of the chain where the substituted carbons get the least number. So here the substitution at 3 and 4 carbons is correct rather than at 4 and 5.

c. Then coming to the substituents, when there are more than one type of substituents the alphabetical order is followed. So here ethyl comes first and then methyl.

d. Finally when, the same group is present more than once the alkyl name is prefixed with di(for two), tri (for three) and so on. Here we have two methyl groups hence named dimethyl. When we take alphabetical order ‘di’ is not considered as it only indicates the number but ‘m’ in methyl is taken into consideration. Hence, first ethyl is named and then methyl.